|Cost-Effectiveness Tool for Evaluating Interventions to Prevent Mother-to-Child Transmission - Manual and Model (UNAIDS, 2000, 94 p.)|
ARV effectiveness refers to the percentage reduction in transmission in a treatment group compared with an untreated comparison group and is one of the most important determinant of overall cost-effectiveness. The model takes the relative percentage reduction in transmission as an input. Since effectiveness is sometimes expressed in absolute and sometimes in relative percentage terms it is necessary to understand the difference between the two and to be able to convert from one to another.
Relative efficacy: Relative percent is the more common way we think about applying percents. Multiplying the probability of transmission without treatment by the relative percent reduction would yield the efficacy. For example, if the intervention is 50% effective and the probability of transmission without treatment is 25%, the relative efficacy is 12.5%.
Absolute efficacy: The amount of reduction expressed directly as percentage points of transmission. For example, if an intervention has an absolute efficacy of 9%, the probability of transmission is reduced from 25% (to use the same background transmission rate as in the previous example) to 16%.
Influence on cost-effectiveness: High. ARV efficacy will have a large effect on program benefits and therefore on cost-effectiveness.
Expected effort of data collection: Low. Published efficacy estimates from the clinical trials are the accepted standard for these inputs. These estimates may be revised as the results from additional trials become available. We cannot foresee any circumstances under which it would be sensible to alter these estimates outside the context of the trials.
C91 - Q91. Relative efficacy. In the blue cells C91 through Q91 the relative reduction in HIV transmission for the ACTG 076, HIVNET 012, Thai, Petra-A and Petra-B regimens are shown as 67.5%, 47%, 51%, 50% and 37% respectively. These figures are based on the reported results of the respective clinical trials with the high and low estimates representing the limits of the 95% confidence interval around the point estimate. (See the ARV section of Published studies and other resources on page 76). These cells are locked to prevent casual changes in these values. Unless you have very good reasons to change them, such as new data from trials in your setting, we suggest that you not alter the default values given.
Background Transmission Rates and Efficacy
Only the relative efficacy from the trials are being applied not the absolute values.
A difference in background transmission rates between your setting and that of the clinical trials does not mean that the estimated efficacies from the treatment will be different in the two settings. For example, in absence of specific knowledge to the contrary, it is reasonable to suppose that a regimen which is 50% effective where transmission rates are 30% will also be 50% effective where transmission rates are only 20%.
As new trials are completed their results will be incorporated into subsequent versions of this model. Please visit the UNAIDS web site at http://www.UNAIDS/MTCT/*** for information about what additional regimens are being considered for inclusion.